You can loop a demo in Q3. It is a great way to test system stability. The CPU intesive programs (RC5,CPU Stability Test, Prime95) will definately get the processor heated, but you can not assume that you are stable if this programs do not give errors.
I have been able to run CPU Stability Test or Prime95 for hours on end with no errors, yet if I start a game of HL/Q3/UT my system would hang after just a little while. I had to increase my core voltage up .05v to give me 100% stablility
If you are able loop Q3 for continuously for an hour or more, I think it would be safe to say you system is stable. If not, try the next core voltage.
How to loop Q3 demos:
1)Place the following lines into a standard text editor (notepad):
timedemo 1
Set D1 "demo demo001; set nextdemo vstr D2"
Set D2 "demo demo002; set nextdemo vstr D1"
vstr D1
(This config will loop the built-in demos 1 & 2 that come with the game.)
2)Save this config into your \Quake3\Baseq3 folder as "demoloop.cfg"
3)From the Q3 console, type "exec demoloop.cfg"
The looping will continue until ESC is presssed.